This application is based on and incorporates herein by reference Japanese Patent Application Nos. 2000-183576 filed on Jun. 19, 2000, and 2001-148400 filed on May 17,2001.
1. Field of the Invention
The present invention relates to a vehicular AC generator and a voltage control unit for the same.
2. Description of Related Art
In an alternator, in addition to a power cable for the supply of generated electric power to a battery and miscellaneous electrical loads mounted on a vehicle, there are used several signal cables for signal communication between a device (hereinafter referred to as xe2x80x9cregulatorxe2x80x9d) for controlling a generated voltage of the alternator and the vehicle. Typical of such signal cables are an IG cable for detecting that a key switch provided on the vehicle is turned ON and for keeping the regulator active, an L cable for warning the vehicle driver of a trouble of the alternator or the battery, (in some case a common terminal is used for both IG and L terminals), and an S cable for detecting a battery voltage and for feeding it back to the regulator to monitor whether the battery voltage is in a controlled state within an appropriate range.
Out of these cables, the power cable is essential and cannot be omitted in view of its function, but the S cable can substantially be substituted by directly monitoring the voltage at an alternator output terminal and can be omitted. As to the L cable, the reliability of the alternator is improved to an extent equal to or even longer than the vehicle life and the number of vehicles not requiring warning is increased. Thus, the L cable can also be omitted.
However, as to the IG cable, some substitute means is needed for omitting the IG cable because it is not known when the key switch is turned ON.
For example, when electric power is fed to the regulator continuously and a power transistor is allowed to operate at an extremely low duty ratio, the alternator can start to generate an electric power immediately no matter when the engine may start. However, when the vehicle is left as it is without being used over a long period, the battery will eventually be exhausted by the electric current consumed in a field coil of the alternator.
When the field source is formed by a permanent magnet, not an electromagnet, the generation of power is started inevitably upon rotation of the alternator. However, the voltage control for a permanent magnet type generator is generally difficult and always requires a high operating torque. Thus, the permanent magnet type generator has disadvantage in view of efficiency and is not suitable for a vehicular AC generator.
U.S. Pat. No. 4,901,704 discloses to detect the rotation of an alternator by utilizing a Hall element. However, since the working environment of the alternator is not always good, there also arise such disadvantages as low reliability and high cost.
In an effort to overcome such disadvantages, JP-A-55-127849 and JP-A-6-84598 propose a method wherein a single-phase voltage in a multi-phases AC voltage induced by interlinkage of a magnetic flux with a multi-phases armature coil, which remains in a rotating magnetic pole as a constituent of a rotating-field type alternator is detected and the alternator rotates. That is, they disclose that the function of IG terminal can be substituted by detecting that an engine is started.
However, use of the technique disclosed in the above JP-A-55-127849 and JP-A-6-284598 under the working environment of the alternator which is not always good may result in the alternator being watered with muddy water or a saline solution, with consequent likelihood of such impurities being adhered to part of the armature coil and part of a rectifier, occurrence of leakage current and incapability of signal detection.
In view of these disadvantages, U.S. Pat. No. 5,602,470 proposes a method wherein a compensating resistor is connected to an output end of an armature coil so as to permit signal detection even when leakage of current occurs. It has turned out that for effectively distinguishing between noise and a actual signal by the method, no matter what leakage of current may occur in the armature coil, it is necessary to set the compensating resistor at a fairly small resistance. That is, that the leakage of current is consumed by the resistor of such a small resistance value means that part of an output current is consumed by the resistor during power generation of the alternator, so that the alternator works insufficiently.
Further, U.S. Pat. No. 5,182,511 discloses a technique wherein a potential difference between two-phase terminal voltages in a multi-phases AC voltage is detected by floating from ground. According to this technique, indeed it is possible to surely detect a voltage signal between two phases even when the leakage occurs. However, it was made clear that a reference voltage of a comparator, i.e., the power supply to a comparator, becomes complicated for comparing the floating voltage signal with a predetermined value. Further, once the alternator starts to generate power, an excessive voltage is applied to the comparator and miscellaneous protective means must be provided for ensuring a stable operation, thereby causing a disadvantage in mounting such that the circuit scale becomes too large.
An object of the present invention is to provide a voltage control unit for a vehicular AC generator capable of surely detecting a voltage signal produced between multi-phases coil terminals.
According to the present invention, a vehicular AC generator including a rotor provided with plural field poles, a field winding for magnetizing the field poles, a diode for return of an exciting current, the diode being connected in parallel with the field coil, an armature having a multi-phases coil and adapted to receive a rotating magnetic field created by the rotor and induce an AC voltage, and a control unit which controls an electric current flowing through the field coil and controls an output. vc sage, wherein a resistor connected between one-phase output terminal in the multi-phases coil and a negative pole potential of a vehicular battery, as well as a switch means for connecting and disconnecting at least one-phase output terminal out of the other phases in the multi-phases coil to and from the negative pole potential of the vehicular battery, are provided. Therefore, when the switch means is closed in the event of occurrence of leakage current in the armature coil, the leakage current returns to the vehicular battery through the switch means, so that a voltage based on leakage current is not generated in the voltage across the resistor. That is, a signal produced between the multi-phases coil terminals can be detected distinctly from noise.
When the rotor rotates in the absence of leakage current, an electromotive force caused by residual magnetism in the field poles constituting the rotor is induced in the armature coil. In this case, when the switch means is opened, that is, in the conventional circuit configuration, about 0.7V of a forward voltage drop of diode is present in the circuit composed of the negative-side diode in X phase, the resistor and the armature coil, while in a reverse-biased period of the diode, the diode is cut off and no current flows in the resistor, thereby resulting a half-wave rectifying action. Consequently, the voltage across the resistor becomes an extremely small signal, so that it is difficult to detect the voltage.
However, in the present invention, when the switch means is closed, any factor which brings about a half-wave rectification or a large voltage drop is not present in the closed circuit composed of the switch means, the resistor and the armature coil. Accordingly, the voltage across the resistor is obtained as a large signal, so that the signal is easily detected.
Further, since the switch means is controlled to turn ON and OFF in accordance with the voltage across the resistor, the switch means can be turned ON and OFF irrespective of whether leakage current is present or not. When there is leakage current, it is possible to separate between noise caused by the leakage of current and an actual signal.
The larger the number of rotations of the rotor is, the larger the magnitude of the voltage generates across the resistor is. That is, when the number of rotations of the rotor is not smaller than a predetermined number and the applied voltage has become larger, the switch means is opened. Thus, it is possible to prevent the generation of heat and insulation breakdown by flowing of a large current in the switch means.